Photometric method and apparatus for measuring a liquid&#39;s turbidity, fluorescence, phosphorescence and/or absorption coefficient

ABSTRACT

Apparatus derives a sample liquid property and has a container with an outlet section having an overflow edge at a horizontal sample surface. A light source above the surface generates a probe light beam at a non-zero angle β 1  to a normal to the surface. A detector above the surface detects intensity of light emitted out through the surface along a first detection axis forming a non-zero angle β 1  with the surface. An optical barrier between the probe light beam and the first detection axis blocks reflected or scattered light. An inlet section receives sample liquid and has an opening to the main section beneath the sample surface. A separating member separates the sample surface of the inlet section from the sample surface of the main section.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 11/243,610 filed Oct. 5,2005, now U.S. Pat. No. ______, and incorporated here by reference.

TECHNICAL FIELD

The invention relates to the field of measuring a liquid's property bymeasuring light emitted from said liquid, which is excited by a probelight beam. The invention relates in particular to the field ofnephelometry and the measurement of turbidity, and to measuringfluorescence and measuring phosphorescence and/or absorption. It relatesto a corresponding measuring apparatus, a flow cell for such apparatus,and to a corresponding measuring method.

Such devices and methods find application, e.g., in chemical industry,pharmaceutical industry, semiconductor production, cooling watermonitoring, drinking water and ground water analysis and monitoring.

BACKGROUND OF THE INVENTION

In many turbidimeters and nephelometers known in the art a sample isilluminated through a window. A problem occurring through this is, thatthe window may be contaminated and accordingly loose (some of) itstransparency. It is not easy to distinguish the corresponding loss oflight intensity from the effect of a different turbidity of the sample.

A free-fall arrangement is also known from the art: a sample liquidforming a free-forming jet is illuminated by light, and scattered lightis detected. In such arrangements, windows are not needed, but usually alarge amount of sample liquid is needed for measurements.

From U.S. Pat. No. 5,400,137 a photometric means is known, which allowsto determine fluorescence and scattering of a sample liquid. Two lightbeams of different wavelengths are directed at a surface of said sampleliquid, and a detector detects light emitted thereupon from said sampleliquid. The sample surface is formed in a container and its boundary isformed by means of an edge to be overflown by excessive sample liquid.

In RU Patent 2,235,310 a turbidimeter is presented, in which a sampleliquid contained in a vessel is illuminated through its surface by lightdirected parallel to a surface normal of sample surface, i.e.,illumination takes place through vertically directed light. Detection oflight scattered by the sample liquid also takes place along an axisparallel to a surface normal of the sample surface. Two light emittersare separated from two detectors by vertically aligned opaque partitionshaving horizontal slots, the slots being arranged within the sampleliquid in immediate vicinity of the sample surface. The partitions serveto prevent any repeated reflections from the vessel's bottom and walls.Excess sample liquid overflows an edge of the vessel in immediatevicinity of the spot, at which the light is directed. One problem withsuch an apparatus is that the intensity of detected light will be verylow, since light incidence direction and detection direction areantiparallel to each other. Another problem could be that the samplesurface is not very stable because of the close proximity of theoverflow-edge and the spot, at which the light is directed. But,according to said RU Patent 2,235,310, any kind of instabilities do notat all affect the measurement, because measured intensities (of bothdetectors, upon illumination with each light source) are evaluated in aspecial way.

In JP Patent Document 03-54436 a turbidimeter is disclosed, in whichpolarized light is obliquely directed at the surface of a sample liquidcontained in a container. Scattered light thereupon emitted from thesample liquid is detected polarization-sensitively by a detector locatedoutside the sample liquid, the detector comprising a polarizer. Betweenthe sample liquid and said polarizer a cylinder is arranged forintercepting external light. The container has an edge, which is to beoverflown by excess sample liquid.

SUMMARY OF THE INVENTION

A goal of the invention is to create an apparatus for deriving at leastone property of a sample liquid from measuring emitted light emittedfrom said sample liquid upon irradiation of said sample liquid with aprobe light beam, which has an increased accuracy.

One object of the invention is to provide for such an apparatus, whichhas an increased measurement stability.

Another object of the invention is to provide for such an apparatus,which is of small dimension.

Another object of the invention is to provide for such an apparatus,which requires only relatively small amounts of sample liquid formeasuring.

Another object of the invention is to provide for such an apparatus,which has a short response time.

Another object of the invention is to provide for such an apparatus,which allows for fast and/or time-resolved measurements.

Another object of the invention is to provide for such an apparatus, bymeans of which an absorption coefficient and, in addition, turbidity orfluorescence or phosphorescence can be derived.

Another object of the invention is to provide for a flow cell for usewith such an apparatus.

Another object of the invention is to provide for a corresponding methodfor deriving at least one property of a sample liquid, which has anincreased accuracy.

One object of the invention is to provide for such a method, which hasan increased measurement stability.

Another object of the invention is to provide for such a method, whichrequires only relatively small amounts of sample liquid for measuring.

Another object of the invention is to provide for such a method, whichhas a short response time.

Another object of the invention is to provide for such a method, whichallows for fast and/or time-resolved measurements.

Another object of the invention is to provide for such a method, bymeans of which an absorption coefficient and, in addition, turbidity orfluorescence or phosphorescence can be derived.

These objects are achieved by an apparatus, a flow cell and by a method,respectively, according to the patent claims.

According to the invention, the apparatus for deriving at least oneproperty of a sample liquid from measuring emitted light emitted fromsaid sample liquid upon irradiation of said sample liquid with a probelight beam, comprises

a container for containing said sample liquid, said sample liquidforming a sample surface;

a light source for generating said probe light beam directed, in anangle β1≠0° with respect to a surface normal of said sample surface, atsaid sample surface in a main section of the container;

a first detector adapted to detecting an intensity of said emitted lightemitted through said sample surface in said main section out of saidsample liquid generally along a first detection axis, said firstdetection axis forming an angle γ1≠0° with a surface normal of saidsample surface; and

an optical barrier arranged between said probe light beam and said firstdetection axis adapted to blocking a propagation of light originatingfrom reflection or scattering of said probe light beam at said samplesurface generally along said first detection axis.

The apparatus can be considered a photometric means or a photometricapparatus or a measuring or monitoring or analysing apparatus or means.When a turbidity of said sample liquid is measured, said apparatus canbe named a turbidimeter or a nephelometer.

Optical elements like light source and detector and correspondinglenses, beam splitters and the like can be arranged outside from thesample liquid. They are not exposed to sample liquid and are therefore(practically) not subject to contamination. Furthermore, neither theprobe light beam nor the emitted light to be detected has to pass awindow that would be in contact with sample liquid.

When a probe light beam impinges on a sample surface (at an angle atwhich no total reflection occurs), it not only continues within thesample liquid as a refracted beam, but it is also reflected andscattered at the sample surface. The intensity of such reflected andsuch scattered light is often two or three or even more orders ofmagnitude higher than at the detector. The optical barrier avoids, thatsaid reflected or scattered light reaches the detector, so that it isnot detected and accordingly does not influence the measurement. It hasto be noted that not only the signal-to-noise ratio would otherwise bestrongly decreased, but also, that the intensity, in particular of saidreflected light, varies strongly, when the sample surface changesslightly, e.g., due to a little wave from some vibration. Themeasurement stability would be low.

With β1≠0° and γ1≠0° it is possible to have the probe light beam locallyseparated from the beam of emitted light to be detected and yet be ableto measure emitted light that has undergone no scattering or singlescattering only.

It is possible to use a polarized probe light beam or to use anunpolarized probe light beam. It is also possible to usepolarization-sensitive detection (e.g., by arranging a polarizer beforea photo-sensitive element) or to use a polarization-insensitivedetector.

In one embodiment said container comprises

an outlet section for removing sample liquid from said container, havingat least one opening to said main section located beneath said samplesurface; and

a first separating member for substantially separating a portion of saidsample surface belonging to said outlet section from a portion of saidsample surface belonging to said main section.

Although an apparatus according to the invention may be used foroff-line analysis, wherein a certain volume of sample liquid is filledinto said container and then analysed (no flow during measurement), buttypically, an apparatus according to the invention will be used in-line,i.e., sample liquid flows steadily through said container and iscontinuously (or in intervals) analyzed. In particular, when usedin-line, said first separating member is helpful, because in said outletsection sample liquid is continuously removed from said container. Aremoval of sample liquid usually leads to turbulences, waves or otherdisturbances in said sample surface, thus creating an uneven andunstable sample surface, which is undesirable, because the refraction ofsaid probe light beam is therethrough changed, thus altering themeasuring conditions. Said first separating member avoids, at least to agreat extent, the propagation of such disturbances into said mainsection of said container. Accordingly, a stable sample surface in suchareas, in which said probe light beam and said emitted light (to bedetected) penetrates said sample surface, is achieved.

By letting sample liquid flow from said main section to said outletsection (well) below said sample surface, disturbances of said samplesurface can be minimized.

In one embodiment, said outlet section comprises an edge to be overflownby excessive sample liquid. Through this edge, a height level of saidsample surface is determined, at least in said outlet section and saidmain section.

In one embodiment, said edge describes a closed shape. That shape may befree from corners. It can be round or elliptic.

Said edge may furthermore be shaped or bent in such a way that, whenoverflown by sample liquid, the occurrence of turbulences in the outletsection is minimized.

In another embodiment, said container comprises

an inlet section for receiving sample liquid to be measured, having atleast one opening to said main section located beneath said samplesurface; and

a second separating member for substantially separating a portion ofsaid sample surface belonging to said inlet section from a portion ofsaid sample surface belonging to said main section.

The advantage of this embodiment is similar to the advantage ofabove-described embodiment with said first separating member. Anintroduction of (fresh) sample liquid usually leads to some disturbancesin said sample surface. By means of said second separating member, itcan (largely) be avoided that such disturbances influence themeasurement.

The inlet section may also function as a degas section, in which samplefluid can degas, i.e., the degas section is used for the removal ofundesired gas solved in the sample fluid, typically just by givingsample fluid some time at the sample surface in the degas unit. Thedegas unit may also be put under an underpressure so as to enhance thedegassing.

In another embodiment, said container comprises a third separatingmember, by which said sample surface is substantially divided into onepartial sample surface, at which said probe light beam is directed, andanother partial sample surface, through which said emitted light isemitted generally along said first detection axis. Such a thirdseparating member divides the sample surface in the main section intotwo parts, which leads to a faster stabilization of the sample surfacein the main section, thus allowing for more stable measuring conditions.

In another embodiment, said third separating member comprises saidoptical barrier.

In another embodiment, said optical barrier extends below said samplesurface. It can extend below said sample surface by at least 1 mm or byat least 2 mm or by at least 5 mm or more. This is advantageous, becausethis way, it can be assured that the optical barrier is always in touchwith the sample liquid in the main section. If the optical barrier wouldextend just to the height level of the sample surface or to just abovethe height level of the sample surface, small changes in the heightlevel of the sample surface, e.g., due to vibrations or changes in flowvelocity, could lead to deformations of the sample surface near theoptical barrier due to surface tension effects (capillary forces). Thiscould adversely affect the measurement, at least if the optical barrieris located close to the spot, where the probe light beam enters thesample liquid and/or to the spot, where the emitted light to be detectedleaves the sample fluid.

In another embodiment, said optical barrier comprises a beam trap fortrapping said light originating from reflection or scattering of saidprobe light beam at said sample surface propagating generally along saidfirst detection axis. Such a beam trap minimizes reflections of saidreflected or scattered probe light from disturbing the detection of saidemitted light to be detected.

In another embodiment, said container comprises a beam trap for trappinglight of said probe light beam underneath said sample surface. Light ofsaid probe light beam extending beyond the place where said emittedlight to be detected originates, can be trapped in said beam trap, sothat it does not disturb the measurement. Said beam trap locatedunderneath said sample surface and avoids a further propagation of lightnot needed for exciting said emitted light to be detected.

In another embodiment, the apparatus comprises a reference detector forobtaining a measure for the intensity of said probe light beam. Forcalibration purposes and in order to compensate for intensity variationsof said light source, the (initial) intensity of said probe light beamcan be monitored by said reference detector. Preferably, the intensityof said probe light beam before penetrating said sample surface ismonitored.

In another embodiment, the apparatus furthermore comprises a beamsplitter for extracting from said probe light beam a reference beam tobe detected by said reference detector. By means of said beam splitter,said reference beam can be coupled out of said probe light beam. Theintensity of said reference beam is proportional to the intensity ofsaid probe light beam. Such a beam splitter may, e.g., be aplane-parallel plate or just a piece of glass pane, or a prism.

In another embodiment, the apparatus comprises a second light source forgenerating a second probe light beam directed, in an angle β2≠0° withrespect to a surface normal of said sample surface, at said samplesurface in a main section of the container. Such a second probe lightbeam may be used to derive a second value for said property of saidsample liquid. In addition, it is possible to derive, with goodprecision, two properties of said sample liquid, one of them theabsorption. The one desired property can be derived with increasedaccuracy. In one embodiment the two probe light beams describe differentoptical paths. The two probe light beams may have the same wavelengthsor may have different wavelengths. In one embodiment, for the incidentangles is valid: β1=β2. In particular, said two probe light beams areparallel to each other. It is possible to have two different lightgenerators, e.g., two lasers, for generating said two probe light beams.

It is also possible to implement said second light source by adding anoptical element, e.g., a beam splitter, so as to provide for said twoprobe light beams, e.g., from one laser or from one light bulb.Detection may be provided for by one detector per probe light beam fordetecting scattered light excited by each probe light beam, or by onlyone detector for detecting scattered light excited by any of said probelight beams.

In one such embodiment, said first detector is adapted to detecting anintensity of emitted light emitted from said sample liquid uponirradiation of said sample liquid with said second probe light beam.

In one embodiment, the apparatus comprises

a second detector for detecting an intensity of said emitted lightemitted through said sample surface in said main section out of saidsample liquid generally along a second detection axis, said seconddetection axis forming an angle γ2≠0° with a surface normal of saidsample surface.

It may be valid: γ1=γ2. Said second detection axis may be alignedparallel to said first detection axis. Whereas in the embodiment withonly one (first) detector and two probe light beams, usually, avariation in time of the intensities of said probe light beams will haveto be provided for in order to distinguish detected intensitiesoriginating from excitation with said first or said second probe lightbeam, e.g., by a (mechanical) chopper, this may not be necessary whentwo detectors are provided and the light paths of said two probe lightbeams are sufficiently separated from each other. Thus, a continuousexcitation and detection may be performed, thus leading to a goodsignal-to-noise ratio.

In one embodiment, said second detector is adapted to detecting anintensity of emitted light emitted from said sample liquid uponirradiation of said sample liquid with said first probe light beam.

It is also possible to have a setup with a first and a second probelight beam and a first and a second detector.

In general, said at least one property is a physical or chemicalproperty that can be derived from measuring emitted light emitted fromsaid sample liquid upon irradiation of said sample liquid with a probelight beam. Said emitted light is excited by said probe light beam. Whatis detected, is an excited emission.

In one embodiment, said at least one property comprises at least one ofturbidity, absorption, fluorescence, phosphorescence.

In one embodiment, in which at least two light sources are employedand/or in which at least two detectors are employed, said at least oneproperty comprises absorption and at least one of turbidity,fluorescence, phosphorescence.

In one embodiment, within said sample liquid said probe light beam formsan angle δ1 with 80°≦δ1≦100° with said emitted light to be detected bysaid first detector. In particular, it can be valid 85°≦δ1≦95°, or moreparticularly 87.5°≦δ1≦92.5°. In case that two probe light beams and/ortwo detectors are employed, the same may hold for the correspondingangles.

Said probe light beam and said first detection axis may be arranged inone plane with a surface normal of said sample surface. In case that twoprobe light beams and/or two detectors are employed, the same may holdfor the corresponding beams or axes. This provides for relatively shortpath lengths and, accordingly, to higher detected intensities (lessscattering and less widening).

In one embodiment applies that β1≧45° or β1≧60° or β1=75°±6°. The samemay apply for β2.

In one embodiment applies that γ1≧35° or γ1≧50° or γ1=65°±6°. The samemay apply for γ2.

Light paths of probe light beam and emitted light to be detected may besymmetrical or asymmetrical.

In one embodiment, said first detector and, if provided, also saidsecond detector, has an acceptance angle (opening angle of acceptancecone) of 20°±10°.

In one embodiment, said probe light beam has no divergence.

In one embodiment, said probe light beam has a convergence of at most3°, at most 2° or at most 1.5°.

Said probe light beam may be a collimated light beam.

Said light source may comprise a collimator.

Said light source may comprise a laser, in particular a diode laser.

Said light source may comprise a filament or, in particular, a lightbulb.

In one embodiment, the light of said probe light beam is infrared light.It may be near-infrared light.

The light of said probe light beam may be visible or invisible light. Itmay be narrow-frequency light or broad-band. It may be white light.

In one embodiment, said emitted light has a wavelength within 860 nm±30nm or within 850 nm±10 nm.

In one embodiment, said sample liquid comprises water.

Said sample liquid may be water (an aqueous solution), in particulardrinking water.

In one embodiment, the apparatus further comprises a processorprogrammed to derive said at least property of said sample liquid fromsaid intensity of said emitted light.

Said processor may be programmed to derive at least two properties, inparticular an absorption coefficient α (Lambert-Beer coefficient) and,in addition, a turbidity σ (or scattering coefficient σ) or a value fora fluorescence or a value for a phosphorescence.

Usually, the sample surface is an interface between a gas and saidsample liquid, in particular the gas being ambient gas, in particularair. The gas could also be a protective gas, e.g., Nitrogen or a noblegas.

Said sample surface can also be a liquid-liquid interface, wherein thesecond liquid in addition to said sample liquid should be a liquid oflower specific weight than said sample liquid, which does not mix withsaid sample liquid.

In one embodiment, a bottom outlet for removing sample liquid from saidcontainer is foreseen. It may comprise an opening in the bottom of saidcontainer. And it may comprise a valve.

The flow cell according to the invention for use with an apparatus forderiving at least one property of a sample liquid from measuring emittedlight emitted from said sample liquid upon irradiation of said sampleliquid with a probe light beam comprises a container for containing saidsample liquid forming a sample surface, and said container comprises

a main section, at which said probe light beam is to be directed andthrough which said emitted light is to be emitted out of said sampleliquid for being measured;

an inlet section for receiving sample liquid to be measured, having atleast one opening to said main section located beneath said samplesurface;

an outlet section for removing sample liquid from said container, havingat least one opening to said main section located beneath said samplesurface; and

an optical barrier arranged in said main section adapted to blocking apropagation of light originating from reflection or scattering of saidprobe light beam at said sample surface.

The advantages of such a flow cell correspond to the advantages of thecorresponding measurement apparatus described above.

In one embodiment, said container furthermore comprises a firstseparating member for substantially separating a portion of said samplesurface belonging to said outlet section from a portion of said samplesurface belonging to said main section.

In one embodiment, said container furthermore comprises a secondseparating member for substantially separating a portion of said samplesurface belonging to said inlet section from a portion of said samplesurface belonging to said main section.

In one embodiment, said flow cell comprises a holder for a light source.

In one embodiment, said flow cell comprises a holder for a (first)detector.

The method according to the invention for deriving at least one propertyof a sample liquid comprises the steps of

directing a probe light beam at a sample surface formed by said sampleliquid in an angle β1≠0° with respect to a surface normal of said sampleliquid;

detecting an intensity of emitted light emitted from said sample liquidupon irradiation of said sample liquid with said probe light beamthrough said sample surface in a main section out of said sample liquidgenerally along a first detection axis, said first detection axisforming an angle γ1≠0° with a surface normal of said sample surface

blocking a propagation of light originating from reflection orscattering of said probe light beam at said sample surface generallyalong said first detection axis.

The advantages of the methods correspond to the advantages of thecorresponding measurement apparatuses described above.

In one embodiment, the method furthermore comprises the steps of

receiving in an outlet section sample liquid from said main sectionthrough at least one opening located beneath said sample surface;

removing sample liquid out of said outlet section from said container;

substantially separating a portion of said sample surface belonging tosaid outlet section from a portion of said sample surface belonging tosaid main section.

Sample liquid to be removed from said container is contained in saidoutlet section.

In one embodiment, the method furthermore comprises the steps of

introducing sample liquid to be measured into said container in an inletsection;

receiving in said main section sample liquid from said inlet sectionthrough at least one opening located beneath said sample surface;

substantially separating a portion of said sample surface belonging tosaid inlet section from a portion of said sample surface belonging tosaid main section.

Said inlet section is to receive new (fresh) sample liquid. In saidinlet section sample liquid is received from out of the container.

In one embodiment, the method furthermore comprises the step ofsubstantially dividing said sample surface into one partial samplesurface, at which said probe light beam is directed, and another partialsample surface, through which said emitted light is emitted generallyalong said first detection axis.

By means of the invention it is possible to measure with a fast responsetime, since it is possible to have a small volume for containing sampleliquid to be analysed.

Further preferred embodiments and advantages emerge from the dependentclaims and the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, the invention is described in more detail by means of examplesand the included drawings. The figures show:

FIG. 1 a cross-section of an apparatus according to the invention alongthe plane indicated as I-I in FIG. 2, partially diagrammatic;

FIG. 2 a top-view onto a cross-section of the apparatus of FIG. 1, thecross-section running along the plane indicated as II-II in FIGS. 1 and3;

FIG. 3 a detail of a cross-section of the apparatus of FIG. 1 along theplane indicated as III-III in FIG. 2, partially diagrammatic;

FIG. 4 a cross-section of an apparatus with two light sources, partiallydiagrammatic;

FIG. 5 a sketch of the light paths in an embodiment with one lightsource and two detectors;

FIG. 6 a diagram describing input to a processor and output of theprocessor.

The reference symbols used in the figures and their meaning aresummarized in the list of reference symbols. Generally, alike oralike-functioning parts are given the same or similar reference symbols.The described embodiments are meant as examples and shall not confinethe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a cross-section of an apparatus 1 according to theinvention. A flow cell 2 comprises a container 3 containing a samplefluid 4. A light source 30, e.g. a diode laser, generates light, whichimpinges on a beam splitter 31 thus generating a reference beam 27 to bedetected by a reference detector 32. Light generated by said lightsource 30 is reflected by a mirror 33 and forms a probe light beam 20.Said probe light beam 20 impinges on the sample surface 5 of the samplefluid 4 with an incidence angle β1 with respect to the surface normal28, which is indicated by a thin dashed-dotted line. Some of theintensity of the probe light beam 20 is reflected at the sample surface5 forming a reflected beam 22 and scattered light, the latter notindicated in FIG. 1. An optical barrier 34 provides for a beam trap 35,which absorbs said reflected beam 22. A large portion of the probe lightbeam 20 is refracted at sample surface 5 and enters the sample liquid 4.

Within the sample liquid 4 the probe light beam 20 excites emitted light21. In case that the apparatus 1 is a turbidimeter 1, the emitted light21 is light scattered from particles within the sample fluid 4. If theapparatus 1 is a fluorescence meter 1, the emitted light 21 isfluorescence light excited by the probe beam 20. If the apparatus 1 isan apparatus for measuring phosphorescence, the emitted light 21 isphosphorescence light excited by the probe light beam 20. A part of theemitted light 21 propagates towards the sample surface 5 and isrefracted when exiting the sample fluid 4.

Outside the sample fluid 4 the emitted light 21, which is to bedetected, forms an angle γ1 with the surface normal 28 of the samplesurface 5. Within the sample fluid 4 the refracted probe light beamforms an angle δ1 with the emitted light 21 to be detected by detector37. The surface normal is indicated as a thin dashed-dotted line. Adetector 37 comprising a photo cell and a lens detects the emitted light21. The detector 37 detects light emitted generally along a detectionaxis 23. Light within a detection cone 24 can be detected by detector37. The intensity of the detected light is at least in a firstapproximation proportional to the amount of fluorescence,phosphorescence and scattering, respectively, within the sample fluid 4.Thus, the detected intensity is closely related to the amount offluorescent material, phosphorescent material and scattering particles,respectively, contained in the sample fluid 4.

The incidence of probe light beam 20 on the sample surface and theemission of emitted light 21 to be detected by the detector 37 out ofthe sample fluid takes place in a main section 6 of the container 3. Athird separating member 11 divides the sample surface 5 within the mainsection 6 into, one partial sample surface, at which the probe lightbeam is directed, and another partial sample surface through which saidemitted light is emitted generally along said first detection axis. Thefunction of the third separation member 11, can also be satisfied. Theoptical barrier 34, functions to block the light originating fromreflection or scattering of the probe light beam. However, in theembodiments shown in FIGS. 2, 4 and 5 it takes on a duel role and alsoperforms the function of the third separating member 11 which, asmentioned above, is to substantially divide the sample surface 5.

FIG. 2 shows a top view onto a cross-section of the apparatus 1 of FIG.1, the cross-section running along the plane indicated as II-II in FIGS.1 and 3. In addition to the main section 6, the container 3 comprises aninlet section 7 and an outlet section 8. Fresh sample fluid 4 isreceived through an inlet tube 17 within the inlet section 7. Inletsection 7 functions at the same time as a degas section 18. During thetime when sample fluid 4 is located in the degas section 18, gas whichis possibly solved in the sample fluid 4 may exit the sample fluidthrough the sample surface 5. In addition, contaminations and particlespossibly carried in the sample liquid 4 can sediment in the inletsection 7 or swim on the sample surface in the inlet section 7. Atransport of such contaminations and particles into the main section 6is inhibited, in particular since openings 12 and 12′, through whichsample fluid 4 flows from the inlet section 7 into the main section 6,are located above the bottom of the container 3 in the inlet section 7and below the sample surface 5 (in the inlet section 7). Instead of twoopenings 12,12′, there could also be only one opening or a larger numberof openings.

The spot 25, where the probe light beam 20 penetrates the sample surface5, and the spot 26, where emitted light to be detected by the detector37 penetrates the sample surface 5, are indicated. From the main section6 the sample fluid 4 flows to the outlet section 8 through an opening 13(see also FIG. 1). The outlet section 8 comprises an outlet tube 16having an edge 15, which is overflown by sample fluid 4. Accordingly,the edge 15 determines the height level of the sample surface 5 withinthe container. Sample fluid 4 after flowing over the edge 15 exits thecontainer through the outlet tube 16.

A first separating member 9 separates the main section 6 from the outletsection 8. A second separating member 10 separates the main section 6from the inlet section 7. The cross-section shown in FIG. 1 runs alongthe dashed dotted line indicated by II-II in FIG. 2. In FIGS. 1, 2 and 3the directions of the flow of sample liquid are indicated by smallarrows.

As indicated in FIG. 1, a beam trap 38 is comprised in the container 3for trapping that portion of the light of the probe light beam 20, whichextends beyond the point where the emitted light 21 to be detectedoriginates. The beam trap 38 comprises a member 40, which is integrallyformed with the first separating member 9, a member 41 and a member 42.On the bottom of the main section 6 a flushing opening 19 is provided,in which a valve 14 is arranged, by means of which the connection 19between the main section 6 and the outlet tube 16 can be opened orclosed. Solid material possibly contained in the sample fluid 4, whichwould sediment on the bottom of the container 3 and possibly disturb themeasurement when whirled up, can be removed from the container 3 byflushing the container with the valve 14 open. And also materialpossibly contained in the sample fluid 4, which would swim on the samplesurface 5, in particular in the main section 6, can be removed from thecontainer 3 by flushing the container with the valve 14 open.

FIG. 3 shows a detail of a cross section of the apparatus 1 of FIG. 1along the plane indicated as III-III in FIG. 2. The inlet tube 17 andthe two openings 12 and 12′, as well as the directions of sample fluidflow are indicated in FIG. 3.

FIG. 4 shows another apparatus 1. As far as the container 3 isconcerned, this apparatus 1 is substantially identical with theapparatus shown in FIGS. 1 to 3. But the optical setup is different fromFIGS. 1 to 3. The apparatus 1 of FIG. 4 comprises two light sources 30and 30 b, which generate two probe light beams 20 and 20 b,respectively. A beam splitter 31 extracts two reference beams 27 and 27b from the probe light beams 20 and 20 b, which reference beams aredetected by reference detector 32. The probe light beams 20, 20 bimpinge on the sample surface 5 under angles β1 and β2, respectively,with the surface normal 28. Light reflected or scattered at the samplesurface 5 is blocked by the optical barrier 34, which prevents lightfrom the probe light beam 20 which is scattered at the sample surface 5from being detected by the detector 37. In particular, reflected beams22 and 22 b are trapped in the beam trap 35 of the optical barrier 34.Within the sample fluid, emitted light is generated which, whenrefracted upon leaving the sample fluid, propagates along the detectiondirection 21, forming an angle γ1 with the surface normal 28 of thesample surface. Within the sample the probe light beams 20 and 20 b,respectively, form angles δ1 and δ2, respectively, with the emittedlight. In the embodiment of FIG. 4, both angles δ1 and δ2 areapproximately 90°. Incident angles β1 and β2 20 are chosen substantiallyequal.

Due to the use of two light sources 30 and 30 b it is possible todetermine not only one, but two properties of the sample liquid 4. Thelength of the light path within the sample fluid 4 is different for thefirst probe light beam 20 and the second probe light beam 20 b.Therefore, a value for the turbidity, the fluorescence or thephosphorescence can be corrected for absorption within the sample liquid4, and an absorption coefficient can be determined. In order todistinguish between light emitted upon excitation with the first probelight beam 20 and light emitted upon excitation with the second probelight beam 20 b, the light sources 30 and 30 b can be switched on andoff alternatingly, e.g., by means of a chopper.

It is possible to implement a second light source for generating asecond probe light beam by using one single light generator (laser, bulb. . . ) plus another optical element, e.g., a mirror or a beam splitter.

FIG. 5 shows another embodiment, but with most details of the containernot shown. This embodiment is similar to the embodiments of the FIGS. 1to 4, but it comprises one light source 30 and two detectors 37 and 37b. It is possible to implement a second detector in form of one singlephoto-sensitive element (e.g., photo cell) plus another optical element,e.g., a mirror or a beam splitter. The light source 30 comprises acollimator lens 36. As shown in FIG. 5, the angles δ1 and δ2 can bechosen as δ1=δ2, and the angles γ1 and γ2 may be chosen as γ1=γ2.

Up to the point, from which emitted light 21 to be detected by thedetector 37 is emitted, the probe light beam 20 travels within thesample liquid 4 by a length L1. The emitted light 21 itself travels by alength of L3 within the sample fluid 4. The lengths L1 and L3 areshorter than the corresponding lengths L2 and L4 occurring inconjunction with light finally to be detected by the second detector 37b. Using the Lambert-Beer equation, the absorption of light within thesample fluid 4 can be calculated separately from the intensity of thegeneration of emitted light (scattered light; fluorescence light;phosphorescence light). By means of an apparatus with more than onedetector, e.g., like shown in FIG. 5, is possible to derive theabsorption and turbidity independently from fluorescence orphosphorescence by employing wavelength-selective detectors. Forexample, a color filter may be placed in the light path before thedetector.

A processor can be used for calculating the desired properties of thesample liquid 4 from the detected intensities.

FIG. 6 shows a diagram describing input to processor 50 and output ofprocessor 50. Processor 50 receives a reference intensity I₀ from thereference detector 32, an intensity I₁ from detector 37 and, ifavailable, an intensity I₂ from detector 37 b. If, like in FIG. 4, twolight sources are employed, two reference intensities can be fed toprocessor 50. The intensities input to processor 50 are used in formulaswithin the processor, and the at least one property of the sample liquidis readily calculated. Intensities measured with known sample liquidsmay be used as gauge measurements.

In case of turbidity- and absorption-measurements with an apparatus likeshown in FIG. 5, with δ1=δ2=90°, values for an absorption coefficient αof the sample liquid 4, an integral scattering coefficient σ of thesample liquid 4, and a scattering coefficient σ₉₀ n 90° to the beam ofthe sample liquid 4 can be obtained in a straight-forward manner, e.g.,along the following lines:

I ₁=σ₉₀ I ₀exp[−(α+σ)(L1+L3)]

I ₂=σ₉₀ I ₀exp[−(α+σ)(L2+L4)]

(α+σ)=−ln(I ₁ /I ₂)/(L1+L3−L2−L4)

σ₉₀ =I ₁ /I ₀*exp[(α+σ)(L1+L3)], and

σ₉₀ =I ₂ /I ₀*exp[(α+σ)(L1+L3)],

with I₀=initial intensity, I1=intensity at first detector, I2=intensityat second detector, and L1, L2, L3, L4 optical path lengths as indicatedin FIG. 5.

For even more precise results, it is possible to operate with morerefined formulas. The case of two light sources and one detector like,e.g., shown in FIG. 4, and the case of only one light source and onlyone detector like, e.g., shown in FIGS. 1-3, can be derived analogously.In case of fluorescence and phosphorescence measurements, correspondingequations can be derived analogously.

Typical dimensions of the apparatus are: volume of contained sampleliquid: of the order of 100 ml to 500 ml, can be below 50 ml or below 20ml, but typically above 10 ml; flow rate of sample liquid within thecontainer 1 ml/s to 10 ml/s, can be as low as 0.2 ml/s±0.1 ml/s.

By means of the separating members 9,10,11 of FIGS. 2 and 4 it ispossible to assure a very calm and flat sample surface 5 in the mainsection 6. The fact that the openings 12,12′,13 connecting the sections6,7,8 of the container are arranged below the sample surface 5 alsosupports the formation of a stable sample surface in the main section 6.

For fluorescence and phosphorescence measurements the wavelength(s) ofthe light source has to be chosen suitably.

1. Apparatus for deriving at least one property of a sample liquid frommeasuring emitted light emitted from said sample liquid upon irradiationof said sample liquid with a probe light beam, said apparatus having apreselected plane and an operable state in which the apparatus containssaid sample liquid and is aligned such that a sample surface formed bysaid sample liquid is at least substantially horizontal andsubstantially coincides with said preselected plane, said apparatuscomprising: a container adapted for containing said sample liquid andcomprising a main section containing, when in said operable state, atleast a portion of said sample liquid, and an inlet section forreceiving sample liquid to be measured, said inlet section being notidentical with said main section and containing, when in said operablestate, at least a portion of said sample liquid, and having at least oneopening to said main section located beneath said sample surface; and aseparating member referred to as second separating member, positionedbetween said inlet section and said main section and substantiallyseparating, when in said operable state, a portion of said samplesurface belonging to said inlet section from a portion of said samplesurface belonging to said main section; and each arranged in a definedposition relative to said container: a light source adapted and arrangedfor generating said probe light beam to be directed, when in saidoperable state, from above said preselected plane in an angle β1≠0° withrespect to a surface normal of said preselected plane, along a path atsaid sample surface in said main section of said container; a firstdetector adapted to detect, when in said operable state, an intensity ofsaid emitted light emitted out of said sample liquid through said samplesurface in said main section generally along a first detection axis,said first detection axis forming an angle γ1≠0° with a surface normalof said preselected plane; and an optical barrier arranged between aspot at which, when in said operable state, the path of said probe lightbeam and said preselected plane intersect, and a spot at which, when insaid operable state, said first detection axis and said preselectedplane intersect, and adapted to block, when in said operable state, apropagation of light originating from reflection or scattering of saidprobe light beam at said sample surface towards and into said firstdetector.
 2. Apparatus according to claim 1, said container comprising:an outlet section not identical with said main section and containing,when in said operable state, at least a portion of said sample liquid,for removing, when in said operable state, sample liquid from saidcontainer and having at least one opening to said main section locatedbeneath said sample surface; and a separating member referred to asfirst separating member positioned between said outlet section and saidmain section and substantially separating, when in said operable state,a portion of said sample surface belonging to said outlet section from aportion of said sample surface belonging to said main section. 3.Apparatus according to claim 2, said outlet section comprising anoverflow edge to be overflown, when in said operable state, by excessivesample liquid, said overflow edge being located at said selected plane.4. Apparatus according to claim 3, said overflow edge describing aclosed shape.
 5. Apparatus according to claim 1, said containercomprising within said main section a separating member referred to asthird separating member by means of which, when in said operable state,a portion of said sample surface belonging to said main section issubstantially divided into one partial sample surface comprising a spotat which, when in said operable state, the path of said probe light beamand said preselected plane intersect, and another partial sample surfacecomprising a spot at which, when in said operable state, said firstdetection axis and said preselected plane intersect.
 6. Apparatusaccording to claim 5, wherein said third separating member and saidoptical barrier are realized in one element.
 7. Apparatus according toclaim 1, wherein said optical barrier extends across said preselectedplane.
 8. Apparatus according to claim 1, said optical barriercomprising a beam trap for trapping, when in said operable state, saidlight originating from reflection or scattering of said probe light beamat said sample surface propagating towards and into said first detector.9. Apparatus according to claim 1, said container comprising a beam traplocated and arranged, when in said operable state, below saidpreselected plane and adapted for trapping light of said probe lightbeam underneath said preselected plane.
 10. Apparatus according to claim1, comprising a reference detector adapted and arranged in a definedposition relative to said container for obtaining a measure for theintensity of said probe light beam.
 11. Apparatus according to claim 10,further comprising a beam splitter arranged in a defined positionrelative to said container and, when in said operable state, in the pathof said probe light beam and adapted for extracting, when in saidoperable state, from said probe light beam a reference, said referencedetector being adapted and arranged for detecting, when in said operablestate, said reference beam, said measure for said intensity of saidprobe light beam being derived from the result of said detection of saidreference beam.
 12. Apparatus according to claim 1, comprising a secondlight source adapted and arranged in a defined position relative to saidcontainer for generating a second probe light beam to be directed, whenin said operable state, from above said preselected plane, in an angleβ2≠0° with respect to a surface normal of said preselected plane at saidsample surface in said main section of the container.
 13. Apparatusaccording to claim 12, wherein said first detector is adapted to detect,when in said operable state, an intensity of emitted light emitted fromsaid sample liquid upon irradiation of said sample liquid with saidsecond probe light beam.
 14. Apparatus according to claim 1, comprising:a second detector positioned in a defined position relative to saidcontainer and adapted to detect, when in said operable state, anintensity of said emitted light emitted out of said sample liquidthrough said sample surface in said main section generally along asecond detection axis, said second detection axis forming an angle γ2≠0°with a surface normal of said preselected plane.
 15. Apparatus accordingto claim 14, wherein said second detector is adapted and arranged fordetecting, when in said operable state, an intensity of emitted lightemitted from said sample liquid upon irradiation of said sample liquidwith said first probe light beam.
 16. Apparatus according to claim 1,said at least one property comprising at least one of turbidity,fluorescence, and phosphorescence.
 17. Apparatus according to claim 12,said at least one property comprising absorption and at least one ofturbidity, fluorescence, and phosphorescence.
 18. Apparatus according toclaim 14, said at least one property comprising absorption and at leastone of turbidity, fluorescence, and phosphorescence.
 19. Apparatusaccording to claim 1, wherein, when in said operable state, said probelight beam forms within said sample liquid an angle δ with 80°≦δ1≦100°with said emitted light to be detected by said first detector. 20.Apparatus according to claim 1, wherein said light source is a lightsource for generating a light beam having no divergence.
 21. Apparatusaccording to claim 1, wherein said light source is an infrared lightsource generating an infrared probe light beam.
 22. Apparatus accordingto claim 1, being adapted for deriving, from measuring emitted lightemitted from said sample liquid upon irradiation of said sample liquidwith a probe light beam, at least one property of a sample liquidcomprising water.
 23. Apparatus according to claim 1, further comprisinga processor programmed to derive said at least one property of saidsample liquid from said intensity of said emitted light detected, whenin said operable state.
 24. A flow cell for use with an apparatus forderiving at least one property of a sample liquid from measuring emittedlight emitted from said sample liquid upon irradiation of said sampleliquid with a probe light beam, said flow cell comprising a containerfor containing said sample liquid forming a sample surface, saidcontainer comprising: a main section, at which said probe light beam isto be directed and through which said emitted light is to be emitted outof said sample liquid for being measured; an inlet section for receivingsample liquid to be measured, having at least one opening to said mainsection located beneath said sample surface; an outlet section forremoving sample liquid from said container, having at least one openingto said main section located beneath said sample surface; an opticalbarrier arranged in said main section configured and arranged forblocking a propagation of light originating from reflection orscattering of said probe light beam at said sample surface; and aseparating member for substantially separating said sample liquid insaid inlet section from said sample liquid in said main section.
 25. Theflow cell according to claim 24, said container furthermore comprising afurther separating member for substantially separating a portion of saidsample surface belonging to said outlet section from a portion of saidsample surface belonging to said main section.
 26. The flow cellaccording to claim 24, including a beam trap for trapping light of saidprobe light beam underneath said sample surface.
 27. Method for derivingat least one property of a sample liquid comprising the steps ofproviding in a container comprising a main section and an inlet sectionfor receiving the sample liquid, a sample liquid having an at leastsubstantially horizontally aligned sample surface; generating a probelight beam; directing, from above, said probe light beam along a path ata portion of said sample surface in said main section in a directionforming an angle β≠0° with respect to a surface normal of said sampleliquid; detecting an intensity of emitted light emitted upon irradiationof said sample liquid with said probe light beam from said sample liquidand through said sample surface in said main section and out of saidsample liquid generally along a first detection axis, said firstdetection axis forming an angle γ1≠0° with a surface normal of saidsample surface, by means of a detector detecting an intensity of lightpropagating above said sample surface along said first detection axis;blocking a propagation of light originating from reflection orscattering of said probe light beam at said sample surface towards andinto said detector; supplying sample liquid to be measured to said inletsection; receiving in said main section sample liquid from said inletsection through at least one opening located beneath said samplesurface; and substantially separating a portion of the sample surfacebelonging to said inlet section from said portion of said sample surfacebelonging to said main section.
 28. Method according to claim 27,comprising the step of using a container comprising a separating memberreferred to as second separating member positioned between said inletsection and said main section and substantially separating said portionof said sample surface belonging to said inlet section from said portionof said sample surface belonging to said main section.
 29. Methodaccording to claim 27, furthermore comprising the steps of: using acontainer comprising an outlet section and a first separating memberpositioned between said outlet section and said main section andsubstantially separating a portion of said sample surface belonging tosaid outlet section from a portion of said sample surface belonging tosaid main section; receiving in said outlet section sample liquid fromsaid main section through at least one opening located beneath saidsample surface; and removing sample liquid out of said outlet sectionfrom said container.
 30. Method according to claim 27, furthercomprising the step of: using a container comprising a separating memberreferred to as third separating member positioned in said main sectionand substantially dividing a portion of said sample surface belonging tosaid main section into one partial sample surface comprising a spot atwhich the path said probe light beam and said sample surface intersect,and another partial sample surface comprising a spot at which said firstdetection axis and said sample surface intersect.
 31. Method accordingto claim 27, further comprising the step of: trapping light of saidprobe light beam underneath said sample surface by means of a beam traplocated underneath said sample surface.